An electric power steering apparatus that assist-controls a steering system of a vehicle by using a rotational torque of a motor, applies a driving force of the motor as a steering assist torque to a steering shaft or a rack shaft by means of a transmission mechanism such as gears or a belt through a reduction mechanism. And then, in order to supply a current to the motor so that the motor generates a desired torque, an inverter is used in a motor drive circuit.
A general configuration of a conventional electric power steering apparatus will be described with reference to FIG. 1. As shown in FIG. 1, a column shaft (a steering shaft) 2 connected to a steering wheel (handle) 1, is connected to steered wheels 8L and 8R through reduction gears 3, universal joints 4a and 4b, a rack and pinion mechanism 5, and tie rods 6a and 6b, further via hub units 7a and 7b. Further, the column shaft 2 is provided with a torque sensor 10 for detecting a steering torque of the steering wheel 1, and a motor 20 for assisting the steering force of the steering wheel 1 is connected to the column shaft 2 through the reduction gears 3. Electric power is supplied to a control unit (an ECU) 100 for controlling the electric power steering apparatus from a battery 13, and an ignition key signal is inputted into the control unit 100 through an ignition key 11. The control unit 100 calculates a current command value of an assist (steering assist) command based on a steering torque T detected by the torque sensor 10 and a velocity Vs detected by a velocity sensor 12, and controls a current I supplied to the motor 20 based on a voltage command value E obtained by performing compensation and so on with respect to the current command value in a current control section. Furthermore, it is also possible to receive the velocity Vs from a CAN (Controller Area Network) and so on.
The control unit 100 mainly comprises a CPU (or an MPU or an MCU), and general functions performed by programs within the CPU are shown in FIG. 2.
Functions and operations of the control unit 100 will be described with reference to FIG. 2. As shown in FIG. 2, the steering torque T detected by the torque sensor 10 and the velocity Vs detected by the velocity sensor 12 are inputted into a current command value calculating section 101. The current command value calculating section 101 decides a current command value Iref1 that is the desired value of the current supplied to the motor 20 such as a three-phase motor based on the steering torque T and the velocity Vs and by means of an assist map and so on. The current command value Iref1 is added in an addition section 102A and then the added value is inputted into a current limiting section 103 as a current command value Iref2. A current command value Iref3 that is limited the maximum current, is inputted into a subtraction section 102B, and a deviation Iref4 (=Iref3−Im) between the current command value Iref3 and a motor current value Im that is fed back, is calculated. The deviation Iref4 is inputted into a current control section 104 that performs PI control and so on. The voltage command value E that characteristic improvement is performed in the current control section 104, is inputted into a PWM control section 105. Furthermore, the motor 20 is PWM-driven through an inverter 106 serving as a drive section. The current value Im of the motor 20 is detected by a current detection circuit 120 within the inverter 106 and is fed back to the subtraction section 102B. In general, the inverter 106 uses EFTs as switching elements and is comprised of a bridge circuit of FETs.
Further, a compensation signal CM from a compensation section 110 is added in the addition section 102A, and the compensation of the system is performed by the addition of the compensation signal CM so as to improve a convergence, an inertia characteristic and so on. The compensation section 110 adds a self-aligning torque (SAT) 113 and an inertia 112 in an addition section 114, further adds the result of addition performed in the addition section 114 and a convergence 111 in an addition section 115, and then outputs the result of addition performed in the addition section 115 as the compensation signal CM.
In a case that the motor 20 is a 3-phase (U-phase, V-phase and W-phase) brushless motor, details of the PWM control section 105 and the inverter 106 is a configuration such as shown in FIG. 3. The PWM control section 105 comprises a duty calculating section 105A that calculates PWM duty command values D1˜D6 of three phases according to a given expression based on the voltage command value E, and a gate driving section 105B that switches ON/OFF after driving each gate of FET1˜FET6 by the PWM duty command values D1˜D6. The inverter 106 comprises a three-phase bridge having top and bottom arms comprised of an upper-FET1 and a lower-FET4 of U-phase, top and bottom arms comprised of an upper-FET2 and a lower-FET5 of V-phase, and top and bottom arms comprised of an upper-FET3 and a lower-FET6 of W-phase, and drives the motor 20 by being switched ON/OFF based on the PWM duty command values D1˜D6.
In such a configuration, although it is necessary to measure a drive current of the inverter 106 or a motor current of the motor 20, as one of request items of downsizing, weight saving and cost-cutting of the control unit 100, it is singulation of the current detector 120 (one-shunt type current detection circuit). A one-shunt type current detection circuit is known as the singulation of a current detection circuit, and for example, the configuration of the one-shunt type current detection circuit 120 is shown in FIG. 4 (Japanese Published Unexamined Patent Application No. 2009-131064 A). That is to say, a one-shunt resistor R1 is connected between the bottom arm of the FET bridge and ground (GND), a fall voltage that is caused by the shunt resistor R1 when a current flowed in the FET bridge, is converted into a current value Ima by an operational amplifier (a differential amplifying circuit) 121 and resistors R2˜R4, and further an A/D converting section 122 A/D-converts the current value Ima at a given timing via a filter comprised of a resistor R6 and a capacitor C1 and then outputs a current value Im that is a digital value. Moreover, a reference voltage of “2.5V” is connected to a positive terminal input of the operational amplifier 121 via a resistor R5.
FIG. 5 shows a wiring diagram of a power supply (a battery), the inverter 106, the current detection circuit 120 and the motor 20, and simultaneously shows a current pathway (indicated by a dashed line) during a state that the upper-FET1 of U-phase is turned ON (the lower-FET4 of U-phase is turned OFF), the upper-FET2 of V-phase is turned OFF (the lower-FET5 of V-phase is turned ON), and the upper-FET3 of W-phase is turned OFF (the lower-FET6 of W-phase is turned ON). Further, FIG. 6 shows a current pathway (indicated by a dashed line) during a state that the upper-FET1 of U-phase is turned ON (the lower-FET4 of U-phase is turned OFF), the upper-FET2 of V-phase is turned ON (the lower-FET5 of V-phase is turned OFF), and the upper-FET3 of W-phase is turned OFF (the lower-FET6 of W-phase is turned ON). It is clear from these current pathways of FIG. 5 and FIG. 6 that the total value of phases that the upper-FET is turned ON, appears in the current detection circuit 120 as a detected current. That is, it is possible to detect a U-phase current in FIG. 5, and it is possible to detect the U-phase current and a V-phase current in FIG. 6. This is the same as in the case that the current detection circuit 120 is connected between the top arm of the inverter 106 and the power supply.